Various types of pipe extrusion apparatuses are well known in the prior art. A typical prior art pipe extrusion apparatus is shown schematically in cross-section in FIG. 1. In that apparatus, the extrusion apparatus is a horizontal press shown generally at 10 that includes a horizontal press chamber 112 containing molding material in the form of fine granules or powdered materials 114 that are to be extruded through a target outlet or extrusion orifice 116 in an endplate 118 of the chamber 112. A guide rod 120 positioned coaxially with the longitudinal axis of the press chamber 112 and concentrically within the extrusion orifice 116 extends through the press chamber 112 and out the extrusion orifice 116. A piston member 122 having a piston endcap 124 thereon is positioned within the chamber 112 and surrounds the guide rod 120. When the piston member 122 moves through the press chamber 112 toward the endplate 118, the molding material 114 between the advancing piston endcap 124 and the endplate 118 is forced toward the endplate so that it fills the cavity 113 of the chamber 112 and extrudes through the extrusion orifice 116 around the guide rod 120 to form a pipe. The pipe formed by extruding the material through the orifice around the guide rod has an internal diameter essentially equal to the diameter of the guide rod 120.
While such an extrusion press is not very complex, there are significant drawbacks. In particular, such a press creates excessive variation in the thickness of the extruded pipe due to an upward displacement of the guide rod 120 in the extrusion orifice 116 during the initial stages of compacting the molding material. Early in the extrusion process, when the molding material 114 in the cavity 113 is pushed toward the endplate 118, as the piston member 122 moves toward the endplate 118, and before the molding material completely fills the cavity 113 within the chamber 112 between the piston and the endplate, the molding material initially located at the bottom of chamber is pushed upward against the guide rod 120, thereby causing the guide rod to dislocate from its concentric alignment within the extrusion orifice. When the guide rod 120 dislocates upward, the thickness of the wall of the pipe extruded therearound is no longer uniform. Upward displacement of the guide rod is shown by the arrow A in FIG. 1, and the dislocation of the guide rod causing the thickness of the pipe to vary is show by the different distances .delta..sub.1 and .delta..sub.2. In the cross-section of the pipe 127 shown in FIG. 2, the thickness of the pipe is not uniform, as again represented by the different wall thickness of .delta..sub.1 and .delta..sub.2. Once the guide rod is dislocated from its coaxially aligned position within the chamber 112 and its concentric position within the extrusion orifice 116, it is held in its new, non-concentric, non-aligned position due to the compaction of the molding material therearound at the endplate.
Dislocation of the position of the guide tube and the formation of a sidewall of non-uniform thickness is detrimental to the ultimate goal of producing a hollow pipe with a sidewall of uniform thickness.